Spin treatment method by which a coating solution or agent is coated to a specified thickness at the center portion of a substrate placed on the spindle has been generally adopted for a coating solution coating work for substrates. It is very difficult to coat the coating solution or agent to a uniform thickness.
FIG. 4 is a sectional view showing a conventional coating equipment. In this figure, a spindle is indicated with "1", a substrate "2" is placed on the placing table of said spindle, a nozzle tube "3" is arranged for dropping a coating solution or agent on said substrate "2", a drive motor which can rotate said spindle "1" is indicated with "4", and a cup "5" and an discharge pipe "6" are provided to prevent a coating solution or agent from being splashed toward the vicinity of said placing table plate "1a" (spin head) on the upper part of said spindle "1". The working procedure for coating a coating solution or agent "g" to said subtrate "2" is as follows;
Namely, a subtrate "2" is placed on said placing table plate "1a" (spindle head) of said spindle "1" and is fixed by means of a vacuum adsorption means. With said subtrate "2" rested, a coating solution or agent "g" is dropped to the center of said substrate "2" through said nozzle tube "3". After that, said spindle "1" is rotated at a low speed (for example, 1,000 r.p.m.) by said drive motor "4", thereby causing said coating solution or agent "g" to be spread to the whole surface of said subtrate "2" and said substrate "2" is continuously rotated at a high speed (for example, 5,000 t.p.m.), thereby causing unnecessary amount of said coating solution or agent "g" to be splashed to the periphery of said subtrate and causing said coating solution or agent "g" to be uniform to a specified thickness (for example, about 1 micron) on said subtrate.
If the film of thus-coated solution or agent "g" is carefully observed in details, it can be found that there is some undulation or waves (or uneven distribution) on the coated surface of said substrate in the direction of rotation. The height of this undulation or wave is from 300 .ANG. to 500 .ANG. and is equivalent to 3 to 5% for the coating film thickness. Existence of such undulation and waves may adversely given influences upon generating patterns on wafers and masks. For example, if such patterns are lines, narrow part and wide part may exist in the thickness of such lines. The characteristics of semi-conductor devices may be greatly deteriorated.
When said coating solution or agent is coated on a rectangular substrate by means of said coating equipment, the coated film thickness of said coating solution or agent is almost uniform in the circular area "S1" outside the above undulation or wave as shown in FIG. 5 A (Plan view) and B (Sectional view). However, the coated film thickness of said coating solution or agent "g" is of uniform or may become abnormally thick at the four corners outside said circular area "S1".
Nevertheless, the effective area "S3" has been widening more and more to form patterns on said subtrate "2", it is remarkably difficult to unify the coated film thickness of said coating solution or agent even at the four corner area "S2". The reason why such peripheral sections are influenced by wind which may be generated by rotation and the coating solution or agent "g" may be dried from said four-corner area "S2", thereby causing the viscosity of said solution or agent "g" to become higher and higher. As a result, the resist at said peripheral four-corner area is hardened prior to the other part. The larger the size of a subtrate becomes, the larger this adverse influence becomes.
FIG. 6 illustrates a mechanism of aerial stream generation in the above situation. As adhesivity may exist in the air in a cup, said air in the cup rotates in the same direction as the spin head when said spin head rotate, and the rotation speed of said air may become different according to positions. Namely, the air in the vicinity of the spin head rotates at a speed which nearly the same as that of the spin head, and the rotation speed of the air may be decreased as being parted from the spin head. As the rotation speed of the air is different according to the positions, some inclination of the pressure (atmospheric pressure) is produced. That is, the atmospheric pressure in the vicinity of the spin head is low and it will become higher and higher as parting from the spin head. Therefore, as shown with arrows (X) and (Y) in FIG. 6, air is flown from the vicinity of the internal area at the side wall in the cup to the spin head. For this reason, particles of said coating solution or agent "g" which are sprung back due to eddy aerial stream are carried on in the vicinity of a subtrate in accompanying with said aerial stream and are adhered to said subtrate again.
In case of rectangular substrates, the peripheral area (four-corner area) on the surface of said coating solution or agent "g" is influenced by eddy aerial stream which is generated by rotation, thereby causing said coating solution or agent on said peripheral area (four-corner area) may be hardened and may be built up at the edge portions. Therefore, it is remarkably difficult to secure uniform thickness of coated film of said coating solution and agent "g".
Recently, a coating equipment which is shown in FIG. 7 is available as one of the means for unifying the thickness of coated film. This coating equipment comprises that a cup "5" is placed in the vicinity of the peripheral portion (at all the periphery) of a substrate "2" placed on the spin head 1a" and an exhaust pipe "6" and the exhaust drive system "7" are provided at the bottom of said cup "5". In this equipment, eddy aerial stream produced in said cup "5" due to difference of the atmospheric pressure (to be low pressure in the vicinity of the spin head "1a" and to become higher and higher as parting from the spin head "1a") which is generated in accompanying with the rotation of a subtrate "2" on the spin head 1a" is prevented by compulsorily exhausting said eddy aerial stream by means of a cup arranged in the vicinity of the peripheral portion of said subtrate "2".
However, in order to exhaust air to such a degree that eddy aerial stream, which is explained in FIG. 6, can be prevented, a large capacity of exhausting is requisite (i.e., the exhaust drive system is remarkably large-sized). At the same time, the exhaust loss may be remarkable due to defectives in structure. Therefore, as the exhaust efficiency is very bad, it is very difficult to secure effective exhaust uniformly from the whole periphery of said subtrate "2". In addition, it is also difficult to secure exhaust so that the coating film may not be influenced. Finally, it is very difficult to form the coating film uniformly..